1. Field of the Invention
The present invention relates in general to the field of signal processing, and more specifically to a control system utilizing a nonlinear delta-sigma modulator having a nonlinear feedback model that models a nonlinear process.
2. Description of the Related Art
Many electronic systems utilize nonlinear processes to generate output signals. For example, plant systems, such as servo control systems and power conversion systems, often utilize nonlinear processes. Power conversion systems often utilize a switching power converter to convert alternating current (AC) voltages to direct current (DC) voltages or DC-to-DC. Switching power converters often include a nonlinear energy transfer process to provide power factor corrected energy to a load.
FIG. 1 depicts a plant and control system 100, which includes a switching power converter 102 and a plant 104. Plant 104 is, for example, a servo control system or a power supply system. The switching power converter 102 provides constant voltage power to load 112. The switching power converter 102 operates in accordance with a nonlinear process in discontinuous current mode. A full, diode bridge rectifier 103 rectifies AC voltage Vin(t) provided by AC voltage source 101 to generate a rectified, time-varying input voltage Vx(t). The switch 108 of switching power converter 102 regulates the transfer of energy from the rectified, time-varying input voltage Vx(t), through inductor 110, to capacitor 106. The peak of input current iin is proportionate to the ‘on-time’ of switch 108, and the energy transferred is proportionate to the ‘on-time’ squared. Thus, the energy transfer process is one embodiment of a nonlinear process. In at least one embodiment, control signal CS is a pulse width modulated signal, and the switch 108 is an n-channel field effect transistor that conducts when the pulse width of CS is high. Thus, the ‘on-time’ of switch 108 is determined by the pulse width of control signal CS. Accordingly, the energy transferred is proportionate to a square of the pulse width of control signal CS. Diode 111 prevents reverse current flow into inductor 110. Energy transferred from inductor 110 is stored by capacitor 106. Capacitor 106 has sufficient capacitance to maintain an approximately constant voltage VC while providing current to load 112. In at least one embodiment, the switching power converter 102 is a boost-type converter, i.e. the voltage VC is greater than the peak of input voltage Vx(t).
The plant and control system 100 also includes a switch state controller 114. The switch state controller 114 generates control signal CS with a goal of causing switching power converter 102 to transfer a desired amount of energy to capacitor 106, and, thus, to load 112. The desired amount of energy depends upon the voltage and current requirements of load 112. To provide power factor correction close to one, switch state controller 114 seeks to control the input current iin so that input current iin tracks the input voltage Vx(t) while holding the capacitor voltage VC constant.
The process of transferring energy from inductor 110 to capacitor 106 represents a nonlinear process. The peak of input current iin is proportionate to the pulse width of the control signal CS, i.e. the ‘on’ (conductive) time of switch 108, and the energy transferred to capacitor 106 is proportionate to the square of the pulse width of the control signal CS and inversely proportionate to the period of control signal CS. Thus, the energy transfer process between inductor 110 and capacitor 106 is inherently nonlinear. Because the energy transfer process of switching power converter 102 is nonlinear, generation of control signal CS to maintain power correction, efficiency, and stable output power is inherently more difficult.